Oil sealed, mechanical rotating vacuum vane pumps having been utilized extensively in the past, both as primary pumps for vacuum loads of 10.sup.-14 -10.sup.-1 mm Hg, or as fore pumps, in combination with diffusion pumps for vacuum loads of greater than 10.sup.-4 mm Hg. Such pumps usually include a pump housing located within a casing. The housing has a vacuum inlet adapted to be connected to a region from which gas is to be pumped for evacuation purposes by a conduit that extends through the casing. The casing includes an oil pool in which the pump housing is immersed. The housing includes a rotary vane pumping structure connected to the inlet and which pumps gases from the region. A drive shaft for the rotary vane pumping structure is connected to and aligned with a motor output shaft. An oil flow path between the pool and the interior of the housing supplies oil to moving parts within the housing for lubrication and forms oil seals within the housing to prevent the flow of pumped gases within certain portions of the housing. The oil in the pool has a tendency to be heated as it flows from the pool to the interior of the housing.
In the past, heat exchange systems have been designed for cooling the heated oil. In one prior art device, the oil is cooled by air pumped by a relatively high speed, centrifugal fan mounted on the motor shaft, between the motor and the pump casing. The centrifugal fan forces relatively high pressure air through a horizontally extending slot in the bottom of a shroud in which the fan is located. The horizontally directed, whirling air components are directed over vertically extending fins on the bottom of the pump casing.
To achieve the desired heat exchange between the oil within the casing, and circulating through the housing, the prior art device requires the air to be pumped at relatively high velocity. The high speed centrifugal fan causes the prior art cooling system to be relatively complex and subject to wear.